Many elevator-type tractor-scrapers have an engine located on the tractor portion and another engine located on the scraper portion with power to drive the wheels and elevator taken either individually from separate engines or from a combination of both engines. Typically, the elevator is driven by one or more hydraulic motors which receive fluid from an engine driven pump or pumps. One of the difficulties encountered with such elevating scrapers is that of providing proper power balance or power utilization during loading while providing good mobility and gradability. To achieve adequate mobility and gradibility, a twin engine tractor-scraper should have between 60-40 and 50--50 front-to-rear engine power distribution. However, in certain situations during loading the power demand by the elevator may reach 60 to 70 percent of the total engine power. Thus, it is evident that minimum acceptable elevator power can equal or exceed the available power from a single engine and it is thereby desirable that the elevator be driven by power drawn equally from both engines. However, driving the elevator by power drawn from both engines creates a problem of obtaining equal pump outputs since several factors influence the output of the pumps including differences in the speeds of the engines and leakage of the pumps and motors.
One solution to this problem is taught by the U.S. Pat. No. 3,443,379 wherein the elevator is driven by a pair of hydraulic motors which individually receive fluid from a pair of variable displacement pumps each of which is driven by a separate engine. The displacement of one of the variable displacement pumps is controlled automatically in response to fluid pressure in the conduits connecting the pumps with the motors so that its output displacement matches that of the other variable displacement pump. However, a variable displacement pump and its attendent controls cost 2 to 3 times that of an equivalent fixed displacement pump arrangement and adds significantly to the total cost of the mechanism. The cost differential between these two types of pumps is even more pronounced on large mechanisms where a relatively large pump or a plurality of pumps are required to provide the necessary flow.